Rammed Earth is Masonry, not Mud!
Digging into the detail: making the case for rammed earth
We all know the stereotypes surrounding earth buildings. The very mention of this material conjures images of gap year students lending their labour in developing economies, Earthships in the deserts of New Mexico, or ‘eco’ visitor centres intended to “promote awareness and understanding of biodiversity and the local environment.” But could this vernacular methodology be a viable low carbon approach in a world of risk-averse clients and contractors’ rigid supply chains?
Most modern multi-storey buildings are now concrete, steel, or increasingly timber post and beam framed structures. Non-structural external walls are then formed with metal studwork or concrete blocks at huge environmental cost. As we move towards 2030 carbon and energy targets, adopt circular economy principles, and engage with the AJ’s own Retrofirst campaign, architects and engineers must commit to greater levels of investigation, testing, and analysis at the very start of a project. Gaining an understanding of the availability and suitability of local materials should begin with exploring the soil beneath our feet, and could lead to savings in both manufacturing and transport emissions.
Rammed earth is not mud, it is masonry
Rammed earth is often seen as primitive or associated with poverty. Yet it can be rendered, clad, painted, plastered, dry-lined, or sealed; and so, used as a direct replacement for energy-hungry clay bricks or concrete blocks. It is formed largely from aggregates such as sands and gravels, with a small clay content acting as a binder. The moisture content must be carefully controlled: too much water will lead to shrinkage and cracking; too little and the constituent materials will not bind together. Nonetheless, it allows for much faster construction than cob or adobe mud buildings. With the right preparation, the material can be used at scale – there is no reason why rammed earth shouldn’t be used for a new office block or housing scheme.
Rammed earth buildings are robust and durable.
Your building will not wash away if you get the details right. Most of the Great Wall of China is rammed earth, the Great Pyramid of Giza too, and there are many examples in the UK of rammed earth buildings in excess of 200 years old. It is true to say that rammed earth is less durable than say, a fired clay brick. But a softwood stud will rot very quickly if not protected from the elements. This is anticipated and understood, and buildings are designed and detailed to account for this. In a similar fashion, earth buildings are typically constructed on a more durable plinth and protected by an increased roof overhang. Alternatively, courses of slate, tile or brick can act as ‘speed bumps’ slowing the flow of water over the wall’s surface and reducing erosion. Natural and man-made admixtures can be introduced to further improve durability.
Earth walls are often relatively thick, but this can be harnessed, providing thermal mass and moisture buffering to regulate internal environments without the usual complex layered approach of modern construction.
Rammed earth is low-carbon
The material has very low embodied energy but needs to be insulated in ‘northern’ climates. The density of rammed earth is similar to clay brickwork or concrete blockwork, and the constituent materials are almost identical. As a result, the thermal performance is almost directly comparable. However, earth construction could easily reduce the carbon footprint of a wall by two-thirds.
Rammed earth need not cost the earth
Construction costs are similar to conventional masonry, but the use of site-won or local materials will ensure it remains value for money. Recycled concrete, brick and stone can all be used to form the bulk of the material, perhaps then requiring only small volumes of dried clay to be imported. You can make a very cheap cake if you need only pay for an egg.
Meeting our climate commitments will require new ways of working and thinking. The current linear design approach, ending with contractors and sub-contractors installing mass-produced products will need to be replaced, at least in part, with a greater focus on circularity. Materials will be sourced from what is locally available, and tested, analysed, and certified on a project-by-project basis. In this context, utilising site-won demolition or excavation materials as part of the fabric of a new project seems almost obvious.